Open File Report OF-AR-28
New Mexico Bureau of Geology and Mineral Resources
A division of
New Mexico Institute of Mining and Technology
40
Ar/39Ar Geochronology results
From the Upper Tesuque
Formation in Northern Espanola
Basin
Prepared By:
Lisa Peters
New Mexico Bureau of Geology, Socorro, NM 87801
Prepared For:
Dr. Dan Koning
New Mexico Bureau of Geology, Socorro, NM 87801
Initia l ly prepared as:
NM Geochronology Research
Laboratory Internal Report
NMGRL-IR 365
April 12, 2004
SOCORRO 2007
NEW MEXICO BUREAU OF GEOLOGY AND MINERAL RESOURCES
Peter A. Scholle, Director and State Geologist
a division of
NEW MEXICO INSTITUTE OF MINING AND TECHNOLOGY
Daniel H. López, President
BOARD OF REGENTS
Ex Officio
Bil l Rich ardson, Governor of New Mexico
Michael J. Davis, Superintendent of Public Instruction
Appointed
Ann Murphy Da ily, President, 1999–2004, Santa Fe
Randall E. Horn, Secretary/Treasurer, 1997–2003, Albuquerque
S idney M. Gutierrez, 2001–2007, Albuquerque
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Robert E. Taylor, 1997–2003, Silver City
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W ILLIAM M C I NTOS H , Geochronologist
M ATT HEIZ LER , Geochronologist
LISA PETER S , Argon Laboratory Technician
R IC H A R D E S SER , Argon Laboratory Technician
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EMERITUS
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Plus research associates, graduate students, and undergraduate assistants.
Introduction
Seven samples of the Lower Coarse White tephra series of the upper Tesuque
Formation in the northern Espanola Basin were submitted for dating by Dan Koning of the
NMBMMR. It was noted during the mineral separation process that all samples were
altered. Only well-rounded plutonic k-feldspars and a trace of well-rounded sanidine
crystals were separated. We, therefore, selected for analysis biotite from the two samples
that were considered to be the least altered (090702-CYZ and SCV-946FU-251102-djk).
This report presents results from these samples. This information is briefly summarized in
Table 1.
Table 1. Brief summary of results.
Phase
Location
Age±2σ (M a )
Comments
090702-CYZ
biotite
Espanola Basin
18.3±1.6
Very low radiogenic
yield
SCV-946FU-251102-djk
biotite
Espanola Basin
13.03±0.40
Low radiogenic yield
Sample
40
Ar/39Ar Analytical Methods and Results
The submitted samples were crushed and cleaned with distilled water. The biotite
was separated with standard magnetic separator and handpicking techniques. The mineral
separates were then loaded into aluminum discs and irradiated for 7 hours at the Nuclear
Science Center in College Station, Texas.
The biotite separates were heated in two steps with a CO2 laser. The lower power A
steps were used to drive off the atmospheric Ar and thereby increase the precision of the B
step. The age data are displayed on probability distribution diagrams (Deino and Potts,
1992). Abbreviated analytical methods for the dated samples are given in Table 2, and
details of the overall operation of the New Mexico Geochronology Research Laboratory are
provided in the Appendix. The argon isotopic results are summarized in Tables 1 and 2 and
listed in Tables 3 and 4.
-1-
The 090702-CYZ and SCV-946FU-251102-djk biotite provide low precision data.
The A step 4 0Ar signal sizes of both 090702-CYZ and SCV-946FU-251102-djk are up to 2
orders of magnitude larger than the B steps and are very non-radiogenic (<6.4%, Table 2).
The B steps are more radiogenic but not as radiogenic as typical biotite samples of this age
(<19.5%, 090702-CYZ and <47.3%, SCV-946FU-251102-djk). Unaltered biotite from
mid-Tertiary tuffs commonly have radiogenic yields >90%. These low radiogenic yields
have resulted in age uncertainties one to two orders of magnitude larger than uncertainties of
typical unaltered biotite. Weighted mean ages are calculated from the B steps of all ten
analyzed biotite grains for each sample (18.3±1.6 Ma, 090702-CYZ and 13.03±0.40 Ma,
SCV-946FU-251102-djk).
Discussion
The prevalence of well-rounded feldspar (both plutonc and volcanic) grains in all the
samples submitted from the Coarse White tephra series of the upper Tesuque Formation
indicates reworking of the tephra material. We have therefore assigned the weighted mean
ages calculated for 090702-CYZ and SCV-946FU-251101-djk (18.3±1.6 Ma and
13.03±0.40 Ma, respectively) as maximum ages for the deposition of the upper Tesuque
Formation. We do caution that with radiogenic yields as low as those from these samples
and atmospheric 4 0Ar signal sizes as large as those seen in the A steps, the possibility of
alteration and accompanying Ar loss is very high. This would have the affect of lowering
the apparent ages of the biotites.
-2-
References Cited
Deino, A., and Potts, R., 1990. Single-Crystal 4 0Ar/3 9Ar dating of the Olorgesailie
Formation, Southern Kenya Rift, J. Geophys. Res., 95, 8453-8470.
Deino, A., and Potts, R., 1992. Age-probability spectra from examination of single-crystal
40
Ar/3 9Ar dating results: Examples from Olorgesailie, Southern Kenya Rift, Quat.
International, 13/14, 47-53.
Mahon, K.I., 1996. The New “York” regression: Application of an improved statistical
method to geochemistry, International Geology Review, 38, 293-303.
Samson, S.D., and, Alexander, E.C., Jr., 1987. Calibration of the interlaboratory
40
Ar/3 9Ar dating standard, Mmhb-1, Chem. Geol., 66, 27-34.
Steiger, R.H., and Jäger, E., 1977. Subcommission on geochronology: Convention on the
use of decay constants in geo- and cosmochronology. Earth and Planet. Sci. Lett., 36,
359-362.
Taylor, J.R., 1982. An Introduction to Error Analysis: The Study of Uncertainties in
Physical Measurements,. Univ. Sci. Books, Mill Valley, Calif., 270 p.
-3-
090702-CYZ biotite, B steps
0.1
% Radiogenic
0.01
Moles 39Ar
(x10-14)
1
0.001
40
15
0
5
K/Ca
10
0
Weighted Mean Age = 18.3±1.6 Ma*
Relative Probability
MSWD = 0.58
18.3 ± 1.6, MSWD = 0.58
0
5
10
15
20
25
30
35
40
Age (Ma)
Figure 1. Age probability distribution diagram of 090702-CYZ single crystal biotite.
*2sigma
SCV-946FU-251102-djk biotite, B steps
10
0.1
0.001
40
0
4
2
0
Weighted Mean Age = 13.03±0.40 Ma*
MSWD = 0.87
0
5
10
15
20
25
30
35
40
Age (Ma)
Figure 2. Age probability distribution diagram of SCV-946FU-251102-djk single
crystal biotite. *2sigma
Table 2. Summary of 40Ar/39Ar results and analytical methods
Sample
090702-CYZ
SCV-946-FU-251102-djk
Lab #
54284
54283
Irradiation
NM-170
NM-170
mineral
biotite
biotite
age
analysis
single crystal step-heat
single crystal step-heat
# of crystals
10
10
Age
18.3
13.03
±2σ
1.6
0.40
comments
maximum age
maximum age
Sample preparation and irradiation:
Minerals separated with standard heavy liquid, Franz Magnetic and hand-picking techniques.
Samples were loaded into a machined Al disc and irradiated for 7 hours in D-3 position, Nuclear Science Center, College Station, TX.
Neutron flux monitor Fish Canyon Tuff sanidine (FC-1). Assigned age = 27.84 Ma (Deino and Potts, 1990)
relative to Mmhb-1 at 520.4 Ma (Samson and Alexander, 1987).
Instrumentation:
Mass Analyzer Products 215-50 mass spectrometer on line with automated all-metal extraction system.
Single crystal biotite were step-heated by a 50 watt Synrad CO2 laser.
Reactive gases removed during a 5 minute reaction with 2 SAES GP-50 getters, 1 operated at ~450°C and
1 at 20°C. Gas also exposed to a W filament operated at ~2000°C and a cold finger operated at -140°C.
Analytical parameters:
Electron multiplier sensitivity averaged 1.69 x 10-16 moles/pA.
Total system blank and background averaged 4990, 5.7, 1.9, 1.3, 2.1 x 10-18 moles at masses 40, 39, 38, 37 and 36, respectively for the biotite analyses.
J-factors determined to a precision of ± 0.1% by CO2 laser-fusion of 4 single crystals from each of 4 radial positions around the irradiation tray.
Correction factors for interfering nuclear reactions were determined using K-glass and CaF2 and are as follows:
(40Ar/39Ar)K = 0.00020±0.0003; (36Ar/37Ar)Ca = 0.00028±0.000005; and (39Ar/37Ar)Ca = 0.0007±0.00002.
Table 3. 40Ar/39Ar analytical data.
ID
Power
40
Ar/39Ar
37
Ar/39Ar
(Watts)
090702-CYZ, C14:170, Biotite, single crystal,
A
2 1043.6
0.2633
B
10 188.0
0.3127
Integrated age ± 1σ
n=2
36
Ar/39Ar
(x 10-3)
39
ArK
K/Ca
40
Ar*
(%)
(x 10-15 mol)
Ar
Age
(%)
(Ma)
39
±1σ
(Ma)
J=0.0008183, D=1.00562, NM-170, Lab#=54284-01
3491.5
583.0
1.05
0.176
1.23
1.9
1.6
1.1
8.4
85.7
100.0
17.4
23.2
18.3
5.4
8.2
4.9
090702-CYZ, C14:170, Biotite, single crystal, J=0.0008183, D=1.00562, NM-170, Lab#=54284-02
A
2 1248.0
0.1053
4216.3
5.03
4.8
0.2
86.4
3.0
B
10
92.38
0.3137
272.3
0.794
1.6
12.9 100.0
17.6
Integrated age ± 1σ
n=2
5.83
5.0
6.2
1.9
5.4
090702-CYZ, C14:170, Biotite, single crystal, J=0.0008183, D=1.00562, NM-170, Lab#=54284-03
A
2 1489.4
0.0427
5040.0
3.56
12.0
0.0
79.0
0.2
B
10
69.33
0.1727
197.5
0.948
3.0
15.8 100.0
16.1
Integrated age ± 1σ
n=2
4.51
3.5
7.4
1.6
5.9
090702-CYZ, C14:170, Biotite, single crystal, J=0.0008183, D=1.00562, NM-170, Lab#=54284-04
A
2 1569.9
0.1056
5303.9
2.06
4.8
0.2
80.4
3.8
B
10 140.1
0.0553
427.5
0.502
9.2
9.8 100.0
20.2
Integrated age ± 1σ
n=2
2.56
7.1
7.8
3.0
6.4
090702-CYZ, C14:170, Biotite, single crystal, J=0.0008183, D=1.00562, NM-170, Lab#=54284-05
A
2 2137.9
0.0595
7212.3
4.25
8.6
0.3
77.0
9.8
B
10
80.57
0.1422
229.7
1.27
3.6
15.8 100.0
18.7
Integrated age ± 1σ
n=2
5.52
11.8
10.5
1.3
8.1
090702-CYZ, C14:170, Biotite, single crystal, J=0.0008183, D=1.00562, NM-170, Lab#=54284-06
A
2 426.7
0.1335
1408.9
1.66
3.8
2.4
80.2
15.2
B
10 141.2
0.1222
427.1
0.411
4.2
10.6 100.0
22.0
Integrated age ± 1σ
n=2
2.07
16.6
2.5
3.7
2.2
090702-CYZ, C14:170, Biotite, single crystal, J=0.0008183, D=1.00562, NM-170, Lab#=54284-07
A
2 2745.7
0.1598
9263.4
2.81
3.2
0.3
88.8
12.4
B
10
78.06
0.2894
212.8
0.353
1.8
19.5 100.0
22.3
Integrated age ± 1σ
n=2
3.16
13.5
13.7
4.2
12.2
090702-CYZ, C14:170, Biotite, single crystal, J=0.0008183, D=1.00562, NM-170, Lab#=54284-08
A
2 224.4
0.1399
710.4
0.265
3.6
6.4
71.3
21.2
B
10 113.7
0.2942
325.9
0.107
1.7
15.3 100.0
25.6
Integrated age ± 1σ
n=2
0.371
22.5
5.6
12.6
5.5
090702-CYZ, C14:170, Biotite, single crystal, J=0.0008183, D=1.00562, NM-170, Lab#=54284-09
A
2 391.6
0.2168
1284.8
0.895
2.4
3.1
89.6
17.6
B
10 255.5
0.0457
806.6
0.104 11.2
6.7 100.0
25.2
Integrated age ± 1σ
n=2
1.000
18.4
2.8
13.7
3.0
090702-CYZ, C14:170, Biotite, single crystal, J=0.0008183, D=1.00562, NM-170, Lab#=54284-10
A
2 1000.0
0.0733
3382.9
3.31
7.0
0.0
93.0
0.5
B
10
93.18
0.1890
273.9
0.249
2.7
13.2 100.0
18.0
Integrated age ± 1σ
n=2
3.56
1.7
5.3
5.8
5.0
SCV-946FV-251102-djk, C13:170, Biotite, single crystal, J=0.0008192, D=1.00562, NM-170, Lab#=54283-01
A
2 682.4
0.1224
2286.6
2.82
4.2
1.0
54.8
9.9
3.5
B
10
36.63
0.1654
92.70
2.32
3.1
25.2 100.0
13.62
0.65
Integrated age ± 1σ
n=2
5.14
11.6
2.0
SCV-946FV-251102-djk, C13:170, Biotite, single crystal, J=0.0008192, D=1.00562, NM-170, Lab#=54283-02
A
2 520.4
0.0955
1736.5
3.76
5.3
1.4
73.5
10.7
2.7
B
10
44.62
0.2621
119.9
1.36
1.9
20.6 100.0
13.6
1.1
Integrated age ± 1σ
n=2
5.12
11.4
2.1
SCV-946FV-251102-djk, C13:170, Biotite, single crystal, J=0.0008192, D=1.00562, NM-170, Lab#=54283-03
A
2 2723.6
0.0903
9144.2
4.65
5.6
0.8
32.4
31.4
13.3
B
10
31.58
0.2145
77.64
9.72
2.4
27.4 100.0
12.75
0.23
Integrated age ± 1σ
n=2
14.4
18.8
4.4
SCV-946FV-251102-djk, C13:170, Biotite, single crystal, J=0.0008192, D=1.00562, NM-170, Lab#=54283-04
A
2 503.1
0.0738
1683.6
10.5
6.9
1.1
86.8
8.3
2.5
B
10
24.99
0.3636
52.17
1.60
1.4
38.4 100.0
14.15
0.90
Integrated age ± 1σ
n=2
12.1
9.1
2.2
SCV-946FV-251102-djk, C13:170, Biotite, single crystal, J=0.0008192, D=1.00562, NM-170, Lab#=54283-05
A
2 512.3
0.1218
1717.4
7.79
4.2
0.9
95.4
7.1
2.6
B
10
30.48
0.6484
63.50
0.376
0.79
38.6 100.0
17.3
3.8
Integrated age ± 1σ
n=2
8.16
7.5
2.4
Table 3. 40Ar/39Ar analytical data.
ID
Power
(Watts)
40
Ar/39Ar
37
Ar/39Ar
36
Ar/39Ar
(x 10-3)
39
ArK
(x 10-15 mol)
K/Ca
40
Ar*
(%)
Ar
Age
(%)
(Ma)
39
±1σ
(Ma)
SCV-946FV-251102-djk, C13:170, Biotite, single crystal, J=0.0008192, D=1.00562, NM-170, Lab#=54283-06
A
2 718.6
0.1320
2409.1
4.09
3.9
0.9
95.3
9.8
3.7
B
10
29.13
0.4753
54.32
0.204
1.1
45.0 100.0
19.3
6.7
Integrated age ± 1σ
n=2
4.29
10.3
3.5
SCV-946FV-251102-djk, C13:170, Biotite, single crystal, J=0.0008192, D=1.00562, NM-170, Lab#=54283-07
A
2 747.0
0.0788
2505.5
3.59
6.5
0.9
71.5
9.7
3.8
B
10
23.50
0.1492
48.54
1.43
3.4
39.0 100.0
13.5
1.0
Integrated age ± 1σ
n=2
5.02
10.8
2.7
SCV-946FV-251102-djk, C13:170, Biotite, single crystal, J=0.0008192, D=1.00562, NM-170, Lab#=54283-08
A
2 952.4
0.1055
3192.9
3.69
4.8
0.9
92.6
13.2
4.7
B
10
24.95
0.1779
46.23
0.296
2.9
45.3 100.0
16.6
4.5
Integrated age ± 1σ
n=2
3.99
13.5
4.3
SCV-946FV-251102-djk, C13:170, Biotite, single crystal, J=0.0008192, D=1.00562, NM-170, Lab#=54283-09
A
2 271.8
0.0969
900.4
5.78
5.3
2.1
83.5
8.5
1.4
B
10
20.06
0.3504
35.85
1.14
1.5
47.3 100.0
14.0
1.2
Integrated age ± 1σ
n=2
6.92
9.4
1.2
SCV-946FV-251102-djk, C13:170, Biotite, single crystal, J=0.0008192, D=1.00562, NM-170, Lab#=54283-10
A
2 293.3
0.1369
970.4
6.17
3.7
2.2
93.4
9.7
1.5
B
10
31.55
1.935
73.85
0.436
0.26
31.3 100.0
14.6
3.2
Integrated age ± 1σ
n=2
6.61
10.0
1.4
Isotopic ratios corrected for blank, radioactive decay, and mass discrimination, not corrected for interfering reactions.
Ages calculated relative to FC-1 Fish Canyon Tuff sanidine interlaboratory standard at 27.84 Ma.
Errors quoted for individual analyses include analytical error only, without interfering reaction or J uncertainties.
Integrated age calculated by recombining isotopic measurements of all steps.
Integrated age error calculated by recombining errors of isotopic measurements of all steps.
Plateau and integrated ages incorporate uncertainties in interferring reaction corrections and J factors.
Decay constants and isotopic abundances after Steiger and Jaeger (1977).
Discrimination = 1.00562 ± 0.00081
Correction factors:
(39Ar/37Ar)Ca = 0.0007 ± 2e-05
(36Ar/37Ar)Ca = 0.00028 ± 5e-06
(38Ar/39Ar)K = 0.01077
(40Ar/39Ar)K = 0.0002 ± 0.0003
Table 4. 40Ar/39Ar analytical data.
ID
40
Ar/39Ar
37
Ar/39Ar
36
Ar/39Ar
-3
(x 10 )
K/Ca
39
ArK
-15
(x 10
mol)
40
Age
(%)
(Ma)
Ar*
090702-CYZ, C14:170, Biotite, single crystal, J=0.0008183, D=1.00562, NM-170, Lab#=54284
03B
69.33
0.1727
197.5
0.458
3.0
02B
92.38
0.3137
272.3
0.384
1.6
10B
93.18
0.1890
273.9
0.120
2.7
05B
80.57
0.1422
229.7
0.611
3.6
04B
140.1
0.0553
427.5
0.243
9.2
06B
141.2
0.1222
427.1
0.198
4.2
07B
78.06
0.2894
212.8
0.172
1.8
01B
188.0
0.3127
583.0
0.085
1.6
09B
255.5
0.0457
806.6
0.050
11.2
08B
113.7
0.2942
325.9
0.054
1.7
Mean age ± 2σ
n=10
MSWD=0.58
4.1 ±6.8
15.8
12.9
13.2
15.8
9.8
10.6
19.5
8.4
6.7
15.3
SCV-946FV-251102-djk, C13:170, Biotite, single crystal,
03B
31.58
0.2145
77.64
07B
23.50
0.1492
48.54
02B
44.62
0.2621
119.9
01B
36.63
0.1654
92.70
09B
20.06
0.3504
35.85
04B
24.99
0.3636
52.17
10B
31.55
1.935
73.85
08B
24.95
0.1779
46.23
05B
30.48
0.6484
63.50
06B
29.13
0.4753
54.32
Mean age ± 2σ
n=10
MSWD=0.87
27.4
39.0
20.6
25.2
47.3
38.4
31.3
45.3
38.6
45.0
16.1
17.6
18.0
18.7
20.2
22.0
22.3
23.2
25.2
25.6
18.3
±1σ
(Ma)
1.6
1.9
5.8
1.3
3.0
3.7
4.2
8.2
13.7
12.6
1.6
J=0.0008192, D=1.00562, NM-170, Lab#=54283
4.696
0.692
0.650
1.118
0.552
0.790
0.211
0.151
0.183
0.102
1.9
2.4
3.4
1.9
3.1
1.5
1.4
0.26
2.9
0.79
1.1
±2.1
12.75
13.5
13.6
13.62
14.0
14.15
14.6
16.6
17.3
19.3
13.03
Notes:
Isotopic ratios corrected for blank, radioactive decay, and mass discrimination, not corrected for interfering reactions.
Ages calculated relative to FC-1 Fish Canyon Tuff sanidine interlaboratory standard at 27.84 Ma.
Errors quoted for individual analyses include analytical error only, without interfering reaction or J uncertainties.
Mean age is weighted mean age of Taylor (1982). Mean age error is weighted error
of the mean (Taylor, 1982), multiplied by the root of the MSWD where MSWD>1, and also
incorporates uncertainty in J factors and irradiation correction uncertainties.
Decay constants and isotopic abundances after Steiger and Jaeger (1977).
Discrimination = 1.00562 ± 0.00081
Correction factors:
(39Ar/37Ar)Ca = 0.0007 ± 2e-05
(36Ar/37Ar)Ca = 0.00028 ± 5e-06
(38Ar/39Ar)K = 0.01077
(40Ar/39Ar)K = 0.0002 ± 0.0003
0.23
1.0
1.1
0.65
1.2
0.90
3.2
4.5
3.8
6.7
0.40
New Mexico Bureau of Mines and Mineral Resources
Procedures of the New Mexico Geochronology Research Laboratory
For the Period June 1998 – present
Matthew Heizler
William C. McIntosh
Richard Esser
Lisa Peters
40
Ar/39Ar and K-Ar dating
Often, large bulk samples (either minerals or whole rocks) are required for K-Ar dating
and even small amounts of xenocrystic, authigenic, or other non-ideal behavior can lead to
inaccuracy. The K-Ar technique is susceptible to sample inhomogeneity as separate aliquots are
required for the potassium and argon determinations. The need to determine absolute quantities
(i.e. moles of 40Ar* and 40K) limits the precision of the K-Ar method to approximately 1% and
also, the technique provides limited potential to evaluate underlying assumptions. In the 40Ar/39Ar
variant of the K-Ar technique, a sample is irradiated with fast neutrons thereby converting 39K to
39
Ar through a (n,p) reaction. Following irradiation, the sample is either fused or incrementally
heated and the gas analyzed in the same manner as in the conventional K-Ar procedure, with one
exception, no argon spike need be added.
Some of the advantages of the 40Ar/39Ar method over the conventional K-Ar technique are:
1. A single analysis is conducted on one aliquot of sample thereby reducing the sample size
and eliminating sample inhomogeneity.
2. Analytical error incurred in determining absolute abundances is reduced by measuring
only isotopic ratios. This also eliminates the need to know the exact weight of the sample.
3. The addition of an argon spike is not necessary.
4. The sample does not need to be completely fused, but rather can be incrementally heated.
The 40Ar/39Ar ratio (age) can be measured for each fraction of argon released and this
allows for the generation of an age spectrum.
The age of a sample as determined with the 40Ar/39Ar method requires comparison of the
measured 40Ar/39Ar ratio with that of a standard of known age. Also, several isotopes of other
elements (Ca, K, Cl, Ar) produce argon during the irradiation procedure and must be corrected
for. Far more in-depth details of the determination of an apparent age via the 40Ar/39Ar method
are given in Dalrymple et al. (1981) and McDougall and Harrison (1988).
2
Analytical techniques
Sample Preparation and irradiation details
Mineral separates are obtained in various fashions depending upon the mineral of
interest, rock type and grain size. In almost all cases the sample is crushed in a jaw crusher and
ground in a disc grinder and then sized. The size fraction used generally corresponds to the
largest size possible which will permit obtaining a pure mineral separate. Following sizing, the
sample is washed and dried. For plutonic and metamorphic rocks and lavas, crystals are
separated using standard heavy liquid, Franz magnetic and hand-picking techniques. For
volcanic sanidine and plagioclase, the sized sample is reacted with 15% HF acid to remove glass
and/or matrix and then thoroughly washed prior to heavy liquid and magnetic separation. For
groundmass concentrates, rock fragments are selected which do not contain any visible
phenocrysts.
The NMGRL uses either the Ford reactor at the University of Michigan or the Nuclear
Science Center reactor at Texas A&M University. At the Ford reactor, the L67 position is used
(unless otherwise noted) and the D-3 position is always used at the Texas A&M reactor. All of
the Michigan irradiations are carried out underwater without any shielding for thermal neutrons,
whereas the Texas irradiations are in a dry location which is shielded with B and Cd. Depending
upon the reactor used, the mineral separates are loaded into either holes drilled into Al discs or
into 6 mm I.D. quartz tubes. Various Al discs are used. For Michigan, either six hole or twelve
hole, 1 cm diameter discs are used and all holes are of equal size. Samples are placed in the 0,
120 and 240° locations and standards in the 60, 180 and 300° locations for the six hole disc. For
the twelve hole disc, samples are located at 30, 60, 120, 150, 210, 240, 300, and 330° and
standards at 0, 90, 180 and 270 degrees. If samples are loaded into the quartz tubes, they are
wrapped in Cu foil with standards interleaved at ~0.5 cm intervals. For Texas, 2.4 cm diameter
discs contain either sixteen or six sample holes with smaller holes used to hold the standards. For
the six hole disc, sample locations are 30, 90, 150, 210, 270 and 330° and standards are at 0, 60,
120, 180, 240 and 300°. Samples are located at 18, 36, 54, 72, 108, 126, 144, 162, 198, 216, 234,
252, 288, 306, 324, 342 degrees and standards at 0, 90, 180 and 270 degrees in the sixteen hole
disc. Following sample loading into the discs, the discs are stacked, screwed together and sealed
3
in vacuo in either quartz (Michigan) or Pyrex (Texas) tubes.
Extraction Line and Mass Spectrometer details
The NMGRL argon extraction line has both a double vacuum Mo resistance furnace and
a CO2 laser to heat samples. The Mo furnace crucible is heated with a W heating element and the
temperature is monitored with a W-Re thermocouple placed in a hole drilled into the bottom of
the crucible. A one inch long Mo liner is placed in the bottom of the crucible to collect the
melted samples. The furnace temperature is calibrated by either/or melting Cu foil or with an
additional thermocouple inserted in the top of the furnace down to the liner. The CO2 laser is a
Synrad 10W laser equipped with a He-Ne pointing laser. The laser chamber is constructed from a
3 3/8” stainless steel conflat and the window material is ZnS. The extraction line is a two stage
design. The first stage is equipped with a SAES GP-50 getter, whereas the second stage houses
two SAES GP-50 getters and a tungsten filament. The first stage getter is operated at 450°C as is
one of the second stage getters. The other second stage getter is operated at room temperature
and the tungsten filament is operated at ~2000°C. Gases evolved from samples heated in the
furnace are reacted with the first stage getter during heating. Following heating, the gas is
expanded into the second stage for two minutes and then isolated from the first stage. During
second stage cleaning, the first stage and furnace are pumped out. After gettering in the second
stage, the gas is expanded into the mass spectrometer. Gases evolved from samples heated in the
laser are expanded through a cold finger operated at -140°C and directly into the second stage.
Following cleanup, the gas in the second stage and laser chamber is expanded into the mass
spectrometer for analysis.
The NMGRL employs a MAP-215-50 mass spectrometer which is operated in static
mode. The mass spectrometer is operated with a resolution ranging between 450 to 600 at mass
40 and isotopes are detected on a Johnston electron multiplier operated at ~2.1 kV with an
overall gain of about 10,000 over the Faraday collector. Final isotopic intensities are determined
by linear regression to time zero of the peak height versus time following gas introduction for
each mass. Each mass intensity is corrected for mass spectrometer baseline and background and
the extraction system blank.
Blanks for the furnace are generally determined at the beginning of a run while the
furnace is cold and then between heating steps while the furnace is cooling. Typically, a blank is
4
run every three to six heating steps. Periodic furnace hot blank analysis reveals that the cold
blank is equivalent to the hot blank for temperatures less than about 1300°C. Laser system
blanks are generally determined between every four analyses. Mass discrimination is measured
using atmospheric argon which has been dried using a Ti-sublimation pump. Typically, 10 to 15
replicate air analyses are measured to determine a mean mass discrimination value. Air pipette
analyses are generally conducted 2-3 times per month, but more often when samples sensitive to
the mass discrimination value are analyzed. Correction factors for interfering nuclear reactions
on K and Ca are determined using K-glass and CaF2, respectively. Typically, 3-5 individual
pieces of the salt or glass are fused with the CO2 laser and the correction factors are calculated
from the weighted mean of the individual determinations.
Data acquisition, presentation and age calculation
Samples are either step-heated or fused in a single increment (total fusion). Bulk samples
are often step-heated and the data are generally displayed on an age spectrum or isochron
diagram. Single crystals are often analyzed by the total fusion method and the results are
typically displayed on probability distribution diagrams or isochron diagrams.
The Age Spectrum Diagram
Age spectra plot apparent age of each incrementally heated gas fraction versus the
cumulative % 39ArK released, with steps increasing in temperature from left to right. Each
apparent age is calculated assuming that the trapped argon (argon not produced by in situ decay
of 40K) has the modern day atmospheric 40Ar/36Ar value of 295.5. Additional parameters for
each heating step are often plotted versus the cumulative %39ArK released. These auxiliary
parameters can aid age spectra interpretation and may include radiogenic yield (percent of 40Ar
which is not atmospheric), K/Ca (determined from measured Ca-derived 37Ar and K-derived
39
Ar) and/or K/Cl (determined from measured Cl-derived 38Ar and K-derived 39Ar). Incremental
heating analysis is often effective at revealing complex argon systematics related to excess
argon, alteration, contamination, 39Ar recoil, argon loss, etc. Often low-temperature heating
steps have low radiogenic yields and apparent ages with relatively high errors due mainly to
5
loosely held, non-radiogenic argon residing on grain surfaces or along grain boundaries. An
entirely or partially flat spectrum, in which apparent ages are the same within analytical error,
may indicate that the sample is homogeneous with respect to K and Ar and has had a simple
thermal and geological history. A drawback to the age spectrum technique is encountered when
hydrous minerals such as micas and amphiboles are analyzed. These minerals are not stable in
the ultra-high vacuum extraction system and thus step-heating can homogenize important details
of the true 40Ar distribution. In other words, a flat age spectrum may result even if a hydrous
sample has a complex argon distribution.
The Isochron Diagram
Argon data can be plotted on isotope correlation diagrams to help assess the isotopic
composition of Ar trapped at the time of argon closure, thereby testing the assumption that
trapped argon isotopes have the composition of modern atmosphere which is implicit in age
spectra. To construct an “inverse isochron” the 36Ar/40Ar ratio is plotted versus the 39Ar/40Ar ratio.
A best fit line can be calculated for the data array which yields the value for the trapped argon
(Y-axis intercept) and the 40Ar*/39ArK value (age) from the X-axis intercept. Isochron analysis is
most useful for step-heated or total fusion data which have a significant spread in radiogenic
yield. For young or low K samples, the calculated apparent age can be very sensitive to the
composition of the trapped argon and therefore isochron analysis should be preformed routinely
on these samples (cf. Heizler and Harrison, 1988). For very old (>Mesozoic) samples or
relatively old sanidines (>mid-Cenozoic) the data are often highly radiogenic and cluster near the
X-axis thereby making isochron analysis of little value.
6
The Probability Distribution Diagram
The probability distribution diagram, which is sometimes referred to as an ideogram, is a
plot of apparent age versus the summation of the normal distribution of each individual analysis
(Deino and Potts, 1992). This diagram is most effective at displaying single crystal laser fusion
data to assess the distribution of the population. The K/Ca, radiogenic yield, and the moles of
39
Ar for each analysis are also often displayed for each sample as this allows for visual ease in
identifying apparent age correlations between, for instance, plagioclase contamination, signal
size and/or radiogenic concentrations. The error (1σ) for each age analysis is generally shown by
the horizontal lines in the moles of 39Ar section. Solid symbols represent the analyses used for
the weighted mean age calculation and the generation of the solid line on the ideogram, whereas
open symbols represent data omitted from the age calculation. If shown, a dashed line represents
the probability distribution of all of the displayed data. The diagram is most effective for
displaying the form of the age distribution (i.e. gaussian, skewed, etc.) and for identifying
xenocrystic or other grains which fall outside of the main population.
Error Calculations
For step-heated samples, a plateau for the age spectrum is defined by the steps indicated.
The plateau age is calculated by weighting each step on the plateau by the inverse of the variance
and the error is calculated by either the method of Samson and Alexander (1987) or Taylor
(1982). A mean sum weighted deviates (MSWD) value is determined by dividing the Chisquared value by n-1 degrees of freedom for the plateau ages. If the MSWD value is outside the
95% confidence window (cf. Mahon, 1996; Table 1), the plateau or preferred age error is
multiplied by the square root of the MSWD.
For single crystal fusion data, a weighted mean is calculated using the inverse of the
variance to weight each age determination (Taylor, 1982). Errors are calculated as described for
the plateau ages above.
Isochron ages, 40Ar/36Ari values and MSWD values are calculated from the regression
results obtained by the York (1969) method.
7
References cited
Dalrymple, G.B., Alexander, E.C., Jr., Lanphere, M.A., and Kraker, G.P., 1981. Irradiation of
samples for 40Ar/39Ar dating using the Geological Survey TRIGA reactor. U.S.G.S., Prof.
Paper, 1176.
Deino, A., and Potts, R., 1990. Single-Crystal 40Ar/39Ar dating of the Olorgesailie Formation,
Southern Kenya Rift, J. Geophys. Res., 95, 8453-8470.
Deino, A., and Potts, R., 1992. Age-probability spectra from examination of single-crystal
40
Ar/39Ar dating results: Examples from Olorgesailie, Southern Kenya Rift, Quat.
International, 13/14, 47-53.
Fleck, R.J., Sutter, J.F., and Elliot, D.H., 1977. Interpretation of discordant 40Ar/39Ar age-spectra
of Mesozoic tholieiites from Antarctica, Geochim. Cosmochim. Acta, 41, 15-32.
Heizler, M. T., and Harrison, T. M., 1988. Multiple trapped argon components revealed by
40
Ar/39Ar analysis, Geochim. Cosmochim. Acta, 52, 295-1303.
Mahon, K.I., 1996. The New “York” regression: Application of an improved statistical method
to geochemistry, International Geology Review, 38, 293-303.
McDougall, I., and Harrison, T.M., 1988. Geochronology and thermochronology by the 40Ar39
Ar method. Oxford University Press.
Samson, S.D., and, Alexander, E.C., Jr., 1987. Calibration of the interlaboratory 40Ar/39Ar dating
standard, Mmhb-1, Chem. Geol., 66, 27-34.
Steiger, R.H., and Jäger, E., 1977. Subcommission on geochronology: Convention on the use of
decay constants in geo- and cosmochronology. Earth and Planet. Sci. Lett., 36, 359-362.
Taylor, J.R., 1982. An Introduction to Error Analysis: The Study of Uncertainties in Physical
Measurements,. Univ. Sci. Books, Mill Valley, Calif., 270 p.
York, D., 1969. Least squares fitting of a straight line with correlated errors, Earth and Planet.
Sci. Lett.., 5, 320-324.
8